• Eric W. Biederman's avatar
    fs: Limit sys_mount to only request filesystem modules. · 7f78e035
    Eric W. Biederman authored
    Modify the request_module to prefix the file system type with "fs-"
    and add aliases to all of the filesystems that can be built as modules
    to match.
    
    A common practice is to build all of the kernel code and leave code
    that is not commonly needed as modules, with the result that many
    users are exposed to any bug anywhere in the kernel.
    
    Looking for filesystems with a fs- prefix limits the pool of possible
    modules that can be loaded by mount to just filesystems trivially
    making things safer with no real cost.
    
    Using aliases means user space can control the policy of which
    filesystem modules are auto-loaded by editing /etc/modprobe.d/*.conf
    with blacklist and alias directives.  Allowing simple, safe,
    well understood work-arounds to known problematic software.
    
    This also addresses a rare but unfortunate problem where the filesystem
    name is not the same as it's module name and module auto-loading
    would not work.  While writing this patch I saw a handful of such
    cases.  The most significant being autofs that lives in the module
    autofs4.
    
    This is relevant to user namespaces because we can reach the request
    module in get_fs_type() without having any special permissions, and
    people get uncomfortable when a user specified string (in this case
    the filesystem type) goes all of the way to request_module.
    
    After having looked at this issue I don't think there is any
    particular reason to perform any filtering or permission checks beyond
    making it clear in the module request that we want a filesystem
    module.  The common pattern in the kernel is to call request_module()
    without regards to the users permissions.  In general all a filesystem
    module does once loaded is call register_filesystem() and go to sleep.
    Which means there is not much attack surface exposed by loading a
    filesytem module unless the filesystem is mounted.  In a user
    namespace filesystems are not mounted unless .fs_flags = FS_USERNS_MOUNT,
    which most filesystems do not set today.
    Acked-by: 's avatarSerge Hallyn <serge.hallyn@canonical.com>
    Acked-by: 's avatarKees Cook <keescook@chromium.org>
    Reported-by: 's avatarKees Cook <keescook@google.com>
    Signed-off-by: 's avatar"Eric W. Biederman" <ebiederm@xmission.com>
    7f78e035
super.c 8.54 KB